Backpressure
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Registered User
Joined: Jan 2003
Posts: 16
From: Nebraska
Backpressure
I recently put a different y pipe and off road pipe on my 97 ws6 and it seemed to make my car run like ***. Im not sure if it's the 02 sims or if its the back pressure. Sure sounds bad *** tho!!! Any ideas?? ive herd that you need back pressure, but people that stip race lt1's run straight headers???? HELP!!!FOR SALE 1997 WS6 
Registered User
Joined: Mar 2002
Posts: 811
From: tallahassee, FL
I put money on the O2 sims, how big is the Y-pipe?
A normal 4-stroke engine will always produce more power with less back pressure:
"No, you absolutely cannot have too little backpressure. That is one of the biggest and dumbest fallacies that exist in relation to 4 stroke IC engines. Why? I'll tell you.
Pumping Losses --- The concept of backpressure means that there will be a high pressure area at the exhaust port. When the valve opens, the escaping exhaust gases have to push against that high pressure area. How can a parasitic loss be good for your engine? The only engines that NEED backpressure are various small engine designs, mostly 2 stroke. The problem with them is they have the tendency to be too effective at driving the exhaust out, loosing compression. I assure you that on a big 4 stroke engine such as an SBC you want the exhaust to have the free-est path possible. In fact, it would be ideal to have a low pressure zone at teh exhaust port. A low pressure area would help to pull the exhaust gases out. Freeing up more power that your engine would have wasted pushing the gases out...which brings up the next topic beautifully.
Scavenging --- We've heard this before, but what does it mean in relation to our discussion of exhaust. Easier for me to use an example. So your engine fires and now it's on the way up on the exhaust stroke. Exhaust doesn't come out at a consistent rate, it comes out in pulses. Each pulse is a high pressure area, and as it moves, it leaves alow pressure area behind it. Aha, there's our scavenging. You want that low pressure area to be at it's peak when the exhaust port opens on the next exhaust stroke. Thats another reason why headers make better power than manifolds. besides just flowing better, instead of all the pulses being dumped in a log fighting with each other, the tubular runners allow the exhaust pulses to stay seperate and create a nice low pressure area behind it. This is also where tuned and equal length headers come into play. Tuned headers are sized such that the length of the tube corresponds the speed of the exhaust pulses so that the low pressure area is maximized at certain rpms. No surprise that short headers are better for high rpms than longtubes.
Problems can surface if you use too large of a primary diameter, loss of torque. The morons are quick to spout 'you lost backpressure and thus torque.' Next time you hear that you will smile and know that that person failed physics in high school. The problem with using too large of a primary is this. The exhaust pulse only has so much gas and energy in it. If the tube is too large, the pulse expands to much, losing energy and thus velocity. When it loses velocity, it can potentially stall and stop moving in the tube, or at least slow down. aha! Too large of a header actually CAUSES backpressure, and thus lost power. We feel this power loss as a loss of torque because usually this effect is much more pronounced at low rpms as much less gas is moving.
The same principles apply to the entire exhaust system, from primaries to collectors to pipes to mufflers. I am too tired to explain it all, books have been written on these topics. I have just scratched the surface, but hopefully you all understand a little better why their is no such thing as good backpressure. I know some of this has been a little oversimplified, but it think it gets the message across.
thank you please drive through~~"
Pretty good explnation (sp?) if you ask me.
A normal 4-stroke engine will always produce more power with less back pressure:
"No, you absolutely cannot have too little backpressure. That is one of the biggest and dumbest fallacies that exist in relation to 4 stroke IC engines. Why? I'll tell you.
Pumping Losses --- The concept of backpressure means that there will be a high pressure area at the exhaust port. When the valve opens, the escaping exhaust gases have to push against that high pressure area. How can a parasitic loss be good for your engine? The only engines that NEED backpressure are various small engine designs, mostly 2 stroke. The problem with them is they have the tendency to be too effective at driving the exhaust out, loosing compression. I assure you that on a big 4 stroke engine such as an SBC you want the exhaust to have the free-est path possible. In fact, it would be ideal to have a low pressure zone at teh exhaust port. A low pressure area would help to pull the exhaust gases out. Freeing up more power that your engine would have wasted pushing the gases out...which brings up the next topic beautifully.
Scavenging --- We've heard this before, but what does it mean in relation to our discussion of exhaust. Easier for me to use an example. So your engine fires and now it's on the way up on the exhaust stroke. Exhaust doesn't come out at a consistent rate, it comes out in pulses. Each pulse is a high pressure area, and as it moves, it leaves alow pressure area behind it. Aha, there's our scavenging. You want that low pressure area to be at it's peak when the exhaust port opens on the next exhaust stroke. Thats another reason why headers make better power than manifolds. besides just flowing better, instead of all the pulses being dumped in a log fighting with each other, the tubular runners allow the exhaust pulses to stay seperate and create a nice low pressure area behind it. This is also where tuned and equal length headers come into play. Tuned headers are sized such that the length of the tube corresponds the speed of the exhaust pulses so that the low pressure area is maximized at certain rpms. No surprise that short headers are better for high rpms than longtubes.
Problems can surface if you use too large of a primary diameter, loss of torque. The morons are quick to spout 'you lost backpressure and thus torque.' Next time you hear that you will smile and know that that person failed physics in high school. The problem with using too large of a primary is this. The exhaust pulse only has so much gas and energy in it. If the tube is too large, the pulse expands to much, losing energy and thus velocity. When it loses velocity, it can potentially stall and stop moving in the tube, or at least slow down. aha! Too large of a header actually CAUSES backpressure, and thus lost power. We feel this power loss as a loss of torque because usually this effect is much more pronounced at low rpms as much less gas is moving.
The same principles apply to the entire exhaust system, from primaries to collectors to pipes to mufflers. I am too tired to explain it all, books have been written on these topics. I have just scratched the surface, but hopefully you all understand a little better why their is no such thing as good backpressure. I know some of this has been a little oversimplified, but it think it gets the message across.
thank you please drive through~~"
Pretty good explnation (sp?) if you ask me.
Thread Starter
Registered User
Joined: Jan 2003
Posts: 16
From: Nebraska
The sensors are like 50 bucks man, thats like 3 hours worth of gas in my hog.
The y pipe and off road pipe is all 3" but its got a "S" in it....
Its weird, but i thoght it sould make a hguge difference...
Any ideas on what to try next?
The y pipe and off road pipe is all 3" but its got a "S" in it....
Its weird, but i thoght it sould make a hguge difference...
Any ideas on what to try next?
Registered User
Joined: Apr 2003
Posts: 1,591
From: PA
Yeah, what he said. I didn't read it all
backpressure=bad
low exhaust velocity(mistaken for lack of backpressure)=bad
good exhaust velocity + low backpressure=good.
You need to check your sims and other work. Also beware that louder exhaust will make your car feel slower because your mind is tricked into thinking your at a higher rpm with your old setup. Just a theory i believe in though. Your car may infact be running like crap due to something else.
good luck
-brent
backpressure=bad
low exhaust velocity(mistaken for lack of backpressure)=bad
good exhaust velocity + low backpressure=good.
You need to check your sims and other work. Also beware that louder exhaust will make your car feel slower because your mind is tricked into thinking your at a higher rpm with your old setup. Just a theory i believe in though. Your car may infact be running like crap due to something else.
good luck
-brent
Registered User
Joined: Dec 1999
Posts: 825
From: Houston, TX
I just thought I'd add that O2 sims will not effect how your car drives. All they do is send a signal to the computer saying you have cats and they're functioning fine. If the O2 sims are messed up or not plugged in, you'll just get an SES light, nothing more.
You need O2 sensors on the front, you can't replace those with O2 sims.
my car is running like *** too...
after i put my cutout that is... i need o2 sims bad... (sensors cost too much)
after i put my cutout that is... i need o2 sims bad... (sensors cost too much)
Registered User
Joined: Feb 2003
Posts: 437
From: Cocoa, FL
yeah i agree with the whole confusion in your head about how the car is running "slower" with louder exhaust. i had my car open headers (hooker lt's
) for a couple of days, and i felt like my car was so slow. it was all due to the fact that it was loud as **** at like 1500-2000 and i figured i was on the throttle more than i actually was. it sounded so bad ***, but theres no way in hell i was gonna be able to drive it like that for a long time...
) for a couple of days, and i felt like my car was so slow. it was all due to the fact that it was loud as **** at like 1500-2000 and i figured i was on the throttle more than i actually was. it sounded so bad ***, but theres no way in hell i was gonna be able to drive it like that for a long time...
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detroit1994z28
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